Output the contributors as JSON - #304

See #242.

This is the structured-output half of #242. Workflows that build their own credits, such as release notes or a readme's contributor list, had to parse the rendered comment to get at the accounts. The new contributors output gives them the list as JSON:

[
  { "github": "octocat", "email": null, "wporg": "octo" },
  { "github": null, "email": "co-author@git.wordpress.org", "wporg": "co-author" },
  { "github": null, "email": "someone@example.com", "wporg": null },
  { "github": "newcomer", "email": null, "wporg": null }
]

• Entries come in the order the comment credits them, after @propsbot add and remove commands are applied.
• email is set only for commit authors GitHub can't match to an account. wporg is null for anyone without a linked WordPress.org account, matching the comment's unlinked list.
• The output is set before the comment is posted, so it's there even when posting fails. It's an empty array when there are no contributors.

Each contributor is listed once, under the first way they contributed, which is how the collector already dedupes them. That's why there are no per-contributor roles, which #242 suggested: adding them would mean changing the collector to keep every role.

The release-aggregation mode from #242 isn't part of this PR.

Testing

1. In a workflow, read it with fromJSON( steps.<id>.outputs.contributors ).
2. Check the list against the props comment.

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